Location of gypsum dissolution cavities by seismic reflection surveying

In the north of England, the dissolution of gypsum by groundwater is responsible for many subsidence features where gypsum beds lie within 100m of the ground surface. The subsidence hazard not only threatens housing and other infrastructure, but also requires expensive special precautions for new construction and potentially may sterilize land that would otherwise be suitable for development. Site investigation by borehole drilling is inadequate for identifying cavities in the gypsum beds because a borehole that proves solid gypsum may have missed a cavity by less than one metre. A surface surveying technique that gives areal coverage is required to detect cavities, which may subsequently be proved by targeted drilling. Seismic reflection is the only viable geophysical technique for detecting gypsum dissolution cavities in the depth range of interest (30-120m). In this project, the seismic reflection method will be applied to the detection of gypsum dissolution cavities by acquiring data for a range of near-surface conditions in the areas around Darlington and Ripon. Different acquisition parameters and processing techniques will be compared and evaluated to determine an appropriate strategy for future applications of the seismic reflection method in site investigation. The method is potentially applicable for detection of cavities elsewhere, e.g. salt dissolution cavities (Cheshire), and abandoned mineworkings in coalfield areas at comparable depths.